Generally, unlike a primary battery that is incapable of being recharged, a secondary battery may be repeatedly charged and discharged. Recently, a high power secondary battery using a non-aqueous electrolyte having high energy density has been developed. When one battery cell is packaged into a pack shape, it forms a low capacity battery that may be used as a power source for various portable small electronic devices such as cellular phones, laptop computers, and camcorders. When several tens of battery cells are connected in series or in parallel, a high capacity secondary battery is formed which may be used as a power source for driving motors such as in a hybrid electric automobile.
Such a high capacity secondary battery is typically assembled in series by connecting a plurality of secondary batteries. Each of the plurality of secondary batteries is composed of an electrode assembly, a case, and a cap assembly.
The battery may be fabricated into various shapes such as cylindrical and prismatic shapes, wherein an electrode assembly thereof has a bar of positive and negative electrode sheets and an insulator separator interposed therebetween that is wound (as a jelly roll) and inserted into a case. The case is then mounted with a cap assembly formed with an outer terminal to provide a battery.
The cap assembly of a secondary battery additionally may include a safety apparatus designed to rupture at a predetermined pressure level and allow trapped gas to escape for the purpose of preventing an explosion of the battery. The cap assembly typically includes an outer terminal that is electrically connected to the electrodes of the electrode assembly and a gasket sealing the case.
A secondary battery may be equipped with a protection circuit for detecting excessive current and intercepting it during charge and discharge of the battery. In other words, a safety apparatus is provided to reduce the possibility of explosion or combustion of a battery due to its inner pressure increasing during a chemical reaction in the case of abnormal operation of the protection circuit or unexpected occurrence of irregular reactions in the battery.
A conventional method of preventing an explosion of a battery includes equipping a cap plate on the case with a pressure releasing valve that ruptures at a predetermined pressure level, as disclosed in Japanese Patent laid-open No. 2000-223102. However, this conventional art has a problem in that a battery may be damaged by outside air and water flowing into the battery after the pressure releasing valve ruptures to emit gas from the battery due to singular structure of the release valve. In other words, the conventional valve eventually results in destruction of the battery by immediately rupturing at the predetermined maximum pressure level that it can sustain, and nullifying the air-tightness inside the battery.
This problem decreases the cycle-life of a battery and also reduces the battery characteristics of a high capacity secondary battery for a hybrid electric vehicle (HEV), because a battery for an HEV requires a particularly high output and large capacity. This means that the high capacity secondary battery contains a much greater energy and has a higher change in inner pressure than a regular battery, resulting in easier rupture of a pressure releasing valve installed therein.
Thus, there is a need for a secondary battery with an improved safety valve that allows the inner pressure of a secondary batter to be lowered in stages.